WEATHERING IN A MARINE CLAY DURING POSTGLACIAL T IME

ABSTRACT: Marine clays of SE Norway lifted above sea-level have been subjected to weathering for 8500 years. As a result of this weathering a major part of the quartz, K-feldspar and plagioclase disappeared in the 0.2--0.6/~m fraction. Trioctahedral il ite passed through the sequence: illite--, mixed-layer illite-vermiculite ~ vermiculite--*dissolution. This transformation started at a depth of 3 m, and the 2: 1 layers dissolved in the upper part of the profile. Chlorite was broken down by weathering into finer particles. As a result, chlorite was first removed from the coarser fractions. Dioctahedral il ite in the clay fractions passed through the following trans-formations in the upper part of the profile: illite ~ mixed-layer illite-vermiculite ~ vermiculite--, chloritized vermiculite. Weathering models for the size fractions 0.2-0.6 and 0-2-2 ~m showed that total amounts of dissolved material from these fractions in the upper part of the profile could be calculated as 55 and 38%, respectively. Dioctahedral 2:1 layers were most resistant to weathering, resulting in 75 % dioctahedral phyllosilicates in the 0.2-0.6 /~m fraction in the uppermost part of the profile, in contrast to 30 % dioctahedral il ite in the unweathered sample. This study illustrates the importance of investigating different fractions and not only material finer than 2/~m